This invention relates to a process for determining the coefficient of thermal expansion (CTE) of calcined petroleum coke. The method involves pressing a pellet comprising a mixture of ground petroleum coke and binder pitch, baking the pellet in an inert atmosphere and measuring the expansion of the pellet. This invention results in a fast, low cost determination of coke CTE.
This method is particularly well suited for use in an on-line quality control process. More particularly, this method, because it is rapid and requires only small samples, can be used to quickly monitor the operating conditions in a commercial coker and accordingly maintain optimum coking conditions. Furthermore, the small sample size requirements of this technique permit CTE determinations of research scale coke samples which are too small for standard CTE evaluation techniques.
Needle coke is used in the manufacture of graphite electrodes for the steel industry and is subject to stringent quality criteria regarding its purity and physical properties. A low coefficient of thermal expansion is one of the most critical parameters of coke quality, because it corresponds to the electrodes' thermal shock resistance. Accordingly, it is necessary to know the CTE of coke prior to its use in electrodes. Furthermore, anode coke can be used in the manufacture of electrodes for the aluminum industry. Accordingly, CTE is an important characteristic of anode coke for this use.
The coefficient of thermal expansion is traditionally measured by calcining coke to 1400.degree. C., crushing and mixing the coke with coal tar pitch, extruding a rod, baking it, graphitizing it, machining it, and measuring the CTE by any method known to those skilled in the art. This technique, "extrusion", suffers from three major drawbacks; 1) it requires large samples (at least 1 kg); 2) it is time intensive (at least several days); and 3) it is manpower intensive (many hours of handling).
One method employed for CTE measurement is the "Wilkening Method". Wilkening in the article "Measurement of Thermal Expansion of Petroleum Cokes and Carbon Anodes for Aluminum Production", 19th Biennial Conference on Carbon, State College, Pa., June, 1989, pages 472-473, teaches slow melting of coal tar pitch onto a layer of ground coke, baking this to approximately 1000.degree. C. to solidify the mass, cutting a cube from the coke-pitch mixture and measuring the CTE of the cube. This method, however, is manpower intensive because the cube must be machined out of the solidified mixture and has a long preparation time due to an extremely slow baking cycle (5 days).
A method developed by Downing and Borger reduces the time period necessary to determine coke CTE, "A Rapid, Small Scale Method for the Determination of the Thermal Expansion Coefficient of Needle Coke Using Extruded Artifacts" 17th Biennial Conference on Carbon, Lexington, Ky., June, 1985, pages 362-363. They show that a mixture of ground coke and coal tar pitch can be extruded into rods and baked for a shorter time period and have good correlation with standard CTE determination methods. More particularly, they showed that the extruded rods can be heated at a much faster rate and a graphitization step can be eliminated by increasing the maximum baking temperature to 1300.degree. C. The net result was that the entire baking cycle would be about 6 hours plus cool down. Although this method shortens the turnaround time necessary for measuring coke CTE, it still involves the time and material intensive extrusion of rods.